Radiation detection



il!" f .J Mr... A.,

Sept. 27, 1960 N. E. PEDERSEN ErAL 2,9545477 RADIATION DETECTION Filed Jan. 22, 1959 ./52 Iii-54 i 56 AMP DET.

This application is a loontiiualt-.ion-vin-p'art of :our iprevious application, Serial No. 719 ,0 l2 illed March-,

1953, now :abandoned The invention relates to methods yandv i detection of radiation. lrhoufgh .the invention' is particularly applicable tothed'etection of infrared/ray, it is equally well adapted for fthe Vdetection of other 't i;

photoradiation, such `as ultrvioletrays and ln the best infrared detectors known 'heret'ofo of semiconductive material, vvsuch pas lead sulfi as -a radiation-sensitive device, 'itseletiical c varying in accordance with tt'he 'intnsityio A biasing voltage is` impressed across cell-and variations in thereSulting ured as `an iridieationof the'intensityoftheradration, J A typical lead sulfide celj ,.fh vin'ganaigea A(A) o fflf square centimeter `.and anfimpedance win predue, when ti iasedjat roe w' infrared 'radiation Vof a density f( square centimeter, -anjotp tvsig'nal y volts. klts conversion 'eflici cy A( g `,0 ratio ofloiitpft power (P0) to exciting power rre, ,mayA ibeiateqrfsa by the formula: f j

gam

, gxmerro'- 24x19 i lf v It vmay thus be seen that such a devicefis relatlvel sensitive and 'that its output signalY must arnpliied'ltto many times -its origina'lvalue 'to'befusefulv, f v

This type of detector has theffurther disadvantage that Ethe bias current introducesfa Background vvnoise effect; Moreover, considerable noisefis introduced liiitousuch Vdevices because of signal and tend to obscureit.. The present invention provides an extremely sensitive signals are, `of course, lampliliedfalong vlitrhtheidesired 2"'7.' characteristics oi the 'tuned cavity: (a) .t"he Q of the current barriers-thatis .itmctionsof Y metals having diierent contact potentials.y 'Ihesenoise tional P Patented Sept.. 2?, web

semiconductivefnrateial, the number of electron carriers is changed andthe `'properties of the microwave tuned cavityalso `follow'th'e'se changes. The amount of microwave power Awhich is reflected 'by the tuned cavi-ty is normally balanced by Vreferencefrnioro'wave power which is reected vfrom another similar cavity, the two signals bein-g of equal magnitude 'but l out of phase. 'Iihe second cavity may alsol include .a similar mass of semiconductive material in order `-to compensate for variations in tempenature andcther ambient conditions. A variation of the properties of the Erst cavity due to the radiation produces an imbalance lbetween the power reiiected fnorn the :two cavities. imbalance constitutes the signal which 'is ampliiied by proper equipment. To facilitate lampliiication detection, the incoming radiation may be interrupted cyolioally by a chopper to produce a niodulatedRF. signa,

` Since thetwo'ca'vities are feeding signals out of phase with respect to each other, Ithe reference cavity may be closed at the end, `or both cavities may be provided with openings and the incoming radiant energy may he chopped alternately at the'two cavities, thus producing a pnshfpull operation `and effectively doubling the output signal.

eradiation .produces la two-fold vari-ation of the cavities `is .lowered because of the presence of greater number of `carriers and (b). theelective electrical length offxuthe cavity is changedfbecause Vof the different phase shifts which are produced vby semiconductive materials of ditferentcariercontent. These two jeects Iare additive,

jI'n :the -dravvings: v 4 Figurefl is fa simplified 'schematic diagnam of a radiation detector.einbodyingffeatures of the present inventiongand Figure `2is a 'cih-'art Ashowing the efect of the resonant cavityin lerlhan'cing theoverall sensitivity of the apparatus.

asV both increase the Varriplitude of the unbalance signal.

@The 'illustrativeembodiment of theinvention shown 'in Figui-'e l includes Ia microwave.. generator 10 which genf method and apparatus for detecting infrared gandfother y lrahation'a system wherein thenoise llevel is :greatly reduced .as compared to .previous devices, principallydue to the elimination of t-he conventionaljd-ireet currentbias and the substitution therefor ofrnierowave radiationfas a biasing agency. v yf ln general terms, the invention accomplishes Vfthesefob'f jeotives by immersing a small yamount of semi-conductive material in a microwave field. The^--sjemicenductivema tori-al is supported by a smal-l :piece y'oflloawlo-ss'dieleetic pol-yfoam material yin the M-spfacebetween the wail-'ls' of 5;@

wave cavity, which thas an' opening g1-tiene -end to permit, the radiation to be detectedto-ifrnpingejapon nheysemi: t;

conductive material. When radiant energy -fa'll-s on the crates microwave energy, "forfeir'ample at .a frequency of the order of 10,000.megacyclefst This` microwave ener gy, Whih 'is of constant frequency and .-alniplitude, is coupled'into awwaveguide 12,1:fo'r example a rectangular waveguide, whichjconveys thesignal through a ferrite s'ollato'r 14 which "insuresfpas'sage of energy along 'the waveguide ..12 only in the directinilaway 'from .the .gencrater-10. v

The 'microwave power frorn gtiide l2 enters Ia convenmagic tee comprising the adjacent portion of the waveguide 1.2, a pair .of waveguide sections le, and 2@ extending side/by 's'i'd'e 'atr'ight angles to the lwaveguide 'section 1 2, andai'waveguidesection lt'extending parallel to the sections {13 and 2l) infthe opposite direction from the section 12. This arrangement. functions to divide thei power 'equallyfbe'tweenfthe waveguides 18 and 2@ which lead respectively to a pair of tuned cavities 2.2 eachof which,contains asemiconductor 26. As will be readily understood by those familiar with the operation of va "magic 'teef Aany signal `reflected from the Icavities 22 baclgallon'g'ftlie respective waveguides 13 and Zit is ooupledfint the waveguide'section 210 lin opposing 'phase relationf'thaitis tesay, the *energyV coupled from lthe sec tion18liito thef se`tion '40'is in 'opposition ltothat cou-pled from 'the section in't'o the section 40. Thus, if the 'efltil :from 'e'aChf the /'WO cavities 22 `viS eiqu'al, hl/Se two 'components will cancel and no signal will be fed into the waveguide section :40; instead, all of `the reflected sig'nalwill 'pass'backup'the waveguide section 192 and Vbe absorbed in and dissipated 'by the [ferrite -isolator 14.2 'If, on the other hand, there is a change in the characteristics of one of the two cavities 22 which causes a difference in either the amplitude or the phase of the two signal' reected back along the respective waveguide sections 18 and 20, a signal proportional in amplitude to the difference will be fed into the waveguide section 40.

The two tuned cavities 22 are identical. They each consist of a length of waveguide defined by an inductive iris 24 at one end and a thin metal wall 25 at the other, the spacing between the iris 24 and the wall 25 being approximately one-half wavelength of the waveguide at the frequency of the generator 10. The endwall 25 of each cavity is provided with a small, tuned aperture 25a, which allows radiation to be focused through it onto the semiconductive material 26 inside the cavity. The semiconductive material, for example, is in the form of a cube one millimeter on each side; it is supported at the geometrical center of the cavity by polyfoam dielectric or other low loss tangent and dielectric constant material. The type of semiconductive material to be used and the temperature at which it will care to be held depends on the wavelength of the radiation to be conducted. For infrared, this may be gold-doped, copper-doped or zincdoped germanium.

An optical system, here schematically represented as a pair of lens 30, serves to focus the infrared rays IR on one or the other of the two semiconductors 26, through a chopper 32, which, although not shown in detail, is a combination of mirrors and/ or prisms, at least one of which is oscillated or rotated to cause the infrared rays from the lens 30 to be directed upon first one and then the other of the two semiconductors 26, alternating between them at a predetermined cyclical rate.

When infrared radiation impinges upon one of the semiconductors 26, the infrared photons excite electrons of the semiconductor from the normally filled (valence) band across the energy gap into the normally empty (conduction) band.. These electrons remain in the conduction band for a period of time which is greater than the period of oscillation of the microwave energy, during which time they are accelerated by the microwave field. The mean free path of these electrons is quite small and many collisions will occur, resulting in transfer of energy to the semiconductor lattice in many phases. Microwave energy will thus be absorbed from the microwave iield and converted into thermal energy, changing the amplitude of the reflected signal.

This change in conductivity of the semiconductor will lower the Q of the cavity 22 and allow more power to pass through it when slightly olf resonance. This change in conductivity of the semiconductor will also have a reactive effect on the tuning of the cavities 22, changing the phase of the reected signal.

The change in both amplitude and phase of the signal reflected back along the waveguide section 18 from the cavity upon which radiation is impinging at any instant will, of course, produce an imbalance between the signal energy reflected from the two cavities, the degree of imbalance being a function of thenumber of infrared photons acting upon the semiconductor 26 in a given time interval. The imbalance signal, which is coupled into the waveguide section 40, may then be detected and measured as a measure of the intensity of the radiation.

In the particular apparatus shown in Figure l, the detecting circuit includes a crystal 48 which is mounted in the waveguide section 40 and which recties the imbalance signal. Due to the action of the chopper 32, the output of this crystal detector is a periodic-'waye form having a frequency equal Ato the chopping rate. This signal, which is typically in the audio-frequency range, is fed to an indicating device which illustratively may include an audio-frequency ampliiier 52 selectively responsive to signals at the chopping frequency, and a second detector or rectifier 54 which produces a D C.

tube. The system is balanced for zero reading of the indicating device 56 With the apertures 25a masked. This is done, for example, by adjustment of a tuning screw S8 in one of the cavities 22. Thus, when the apertures are unmasked, the reading of the device 56 will be a function of the intensity of radiation impinging upon the semiconductors 26. l

Even a very `small amount of infrared radiation will produce a largeimbalance of energy in the waveguide section 40 and a substantial reading on the indicator 56. The device is, therefore, extremely sensitive. Assuming a quantum eiiciencyV of unity (that is, that the number of electrons excited into the conduction band is equal to the number of incident photons-a condition which is closely approximated in practice), a conversion eiciency for gold-doped germanium on the order of unity may be calculated. /This means that the overall sensitivity of the present system is roughly l0 million times greater than that of the D.C.-biased lead suliide cell of the prior art.

The elimination of the conventional biasing current avoids the noise introduced by the biasing current. Moreover, the gold-doped germanium can be used at very cold temperatures, for example the temperature of liquid nitrogen. At such temperatures, the thermal noise level is extremely low. Thus, the signal-to-noise ratio of the system is high, and the signal can be amplified to useful levels without a prohibitive level of amplified noise.

The eiect of each of the two resonant cavities 22 on the overall gain of the system is illustrated in Figure 2. In this ligure, the line 60 is a plot of the characteristics of the semiconductor, the ordinate 62 representing units of infrared power and the abscissa 64 representing units of conductivity of the semiconductor. The curve 66 represents the characteristics of the resonant cavity 22 wherein the common abscissa 64 represents the conductivity of the semiconductor and the ordinate 68 represents the transmission coefficient of the cavity 22 (that is, the ratio of reflected power to incident power). The Wave form 70 represents the amplitude of the incident infrared power, as modulated by the chopper 32 (Figure l). This infrared power acts upon the semiconductor 26 (Figure l to produce a periodic change in its conductivity as represented by the wave form 72. These changes in thezconductivity of the semi-conductor 26 produce corresponding changes in the tuning of the cavity 22 and accordingly in its transmission coefficient, as indicated by the wave form 74. The cavity 22 is operated slightly of` resonance, on a portion 66a of the curve 66 Where the curve is comparatively linear and has a very steep slope. This steepness of the slope of the portion 66a of the curve 66 causes the amplitude of the wave form 74 representing the periodic change in transmission coeicient of the cavity to be quite large as compared to the change in conductivity of the semi-conductor as represented by the wave form 72.

In other words, the resonant cavities 22 act as ideal (noise-free) amplifiers with an amplification factor 0f'Q/1r.

Since the two cavities operate in push-pull, the'effective output signal is doubled. When the cavities are adjusted for identical Q as well as center frequency, the system acts to eliminate fluctuations in both frequency and amplitude of the microwave source 10 as seen at the detector 48. Thus, the system can be made to double infrared sensitivity and, at the same time, cancel residual source noise.

Thesystem disclosed is thus suitable for applications Where extremely high sensitivity and low noise level are of paramount importance. For example, it may be used in search systems for detecting aircraft at extreme ranges. Similarly, it may be used in a lire-control system or in a missile-to-missile homing system. It may also be used graphs or for night observation.

Other semiconductors than copper, gold and Zinc @Opf-.d germanium may, of course, be used as the semiconductor 26. For example,` lead sulfide, indium antimonitepand selenium telluride areV possible satisfactory suhStllS, depending upon the4 type of radiation to be detected. The basic requirement is vthat the energy gap of the muv terial be sufficiently small'that electrons `in .the valence band may be caused to jump the-forbidden band Vand enter theV conduction band when` excited at the energy level of the photons to be detected.

As is well known, the energy level of a photon of radiation is proportional toits frequency, being expressed by the formula E=1V, wherein h is Plancks constant and V is the frequency. Thus, in detecting radiation at very short wave lengths, such as ultraviolet, even intrinsic germanium and similar material having a relatively large energy gap may be used.

From the foregoing description, it will be appreciated that the present invention provides a method and apparatus for detecting radiation whichis more sensitive and has a higher signal/noise ratio 'than anysystem known heretofore. However, it should be emphasized that the particular embodiments of the invention which are shown or disclosed herein are intended as merely illustrative of the principles of the invention rather than as restrictive of the scope thereof or of the covering of this patent, which is defined only by the appended claims.

We claim:

l. A radiation detector comprising a radio-frequency generator, a first transmission device coupled to said generator to receive radio-frequency energy therefrom, a semiconductor in energy-absorptive relation to said first transmission device, said semiconductor being arranged in the field of the radiation to be detected, a second transmission device coupled to said generator, a detector arranged to receive radio-frequency energy from said two transmission devices in opposing phase relation, means for balancing the amplitude of the radio-frequency energy transmitted to said detector by said first and second transmission devices in the absence of radiation in said field, whereby said detector will indicate an imbalance of such radio-frequency energy dependent upon the intensity of the radiation in said field.

2. A radiation detector comprising a micro-wave generator, a first waveguide section coupled to said generator to receive microwave energy therefrom, a semi-conductor mounted in said first waveguide section, an optical opening in said first waveguide section for permitting the radiation to be detected to impinge upon said semiconductor, a second waveguide section coupled to said generator to receive microwave energy therefrom, a third waveguide section coupled to said first and second waveguide sections to receive said microwave energy therefrom in opposing phase relation, and a `detector coupled to said third waveguide section to detect the imbalance of the microwave energy received therein from said first and second waveguide sections.

3. A radiation detector comprising a microwave generator, a first waveguide section coupled to said generator to receive microwave energy therefrom, a semiconductor mounted in said first waveguide section, an optical opening in said first waveguide section for permittingthe radiation to be detected to impinge upon said semiconductor, a chopper in the path of said radiation to cause periodic interruption of the excitation of said semiconductor thereby at a predetermined frequency, a second waveguide section coupled to said generator to receive microwave energy therefrom, a third waveguide section coupled to said first and second waveguide sections to receive said microwave energy therefrom in opposing phase relation, and detector means sensitive to microwave signals modulated at said predetermined frequency to detect the periodic imbalance of microwave energy transmitted thereto by said second and third waveguide sections.

4. A radiation detector comprising a microwave generator, a first waveguide section coupled to said generator toreceive microwave energy therefrom, a semiconductor mounted in said first waveguide section, said semiconductor being. arranged in the field of the radiation ,to be detected, a chopper in the path of said radiation to cause periodic interruption of the excitation of said semiconductor thereby at a predetermined frequency, a second wave-- guide coupled to said,y generator to receive microwave energy therefrom, a third waveguide sectionv coupled to said first and second waveguide sections to receive said microwave energy therefrom in opposing phase relation.

a local oscillator coupled toY said third waveguide, a rst detector ccupledtc Said third waveguide to detect auiutermediate frequency product of the frequencies of said generator and local oscillator, an intermediate frequency ampliiicr coupled tc Said. detector for amplifying. such intermediate frequency signal, a second detector coupled to said intermediate frequency ampliiier for detecting, the Vundulating .Signal at SaidhOPPing frequency, an am.-

plieicoupledV to said Second detector and selectively responsive to signals at said predetermined chopping fre.- quency, and indicating means, coupled to the latter said amplifier for indicating the amplitude of the chopping frequency signal.

5. A radiation detector comprising a microwave generator, a rst waveguide section coupled to said generator to receive therefrom microwave energy at a predetermined frequency, a cavity resonant at said frequency coupled to said first waveguide section, a semiconductor mounted in said cavity, an optical opening in said cavity for permitting radiation to impinge upon said semiconductor, a second waveguide section coupled to said generator to receive microwave energy therefrom, a third waveguide section coupled to said first and second waveguide sections to receive said microwave energy therefrom in opposing phase relation, and a detector coupled to said third waveguide section to detect the imbalance of the microwave energy received therein from said first and second waveguide section.

6. A radiation detector comprising a microwave generator, a first waveguide section coupled to said generator to receive therefrom microwave energy at a predetermined frequency, a cavity resonant at said frequency coupled to said first waveguide section, a semiconductor supported near the geometric center of said cavity by a dielectric material, an optical opening in said cavity for permitting radiation to impinge upon said semiconductor, a second waveguide section coupled to said generator to receive microwave energy therefrom, a third waveguide section coupled to said first and second waveguide sections to receive said mircowave energy therefrom in opposing phase relation, means for balancing the amplitude of microwave energy transmitted to said third waveguide section in the absence of radiation, and a detector coupled to said third waveguide section to detect the imbalance of the mircowave energy received therein from said first and second waveguide sections.

7. The invention as claimed in claim 6 wherein said dielectric materialis a foam resin which substantially fills said cavity.

8. A radiation detector comprising a microwave generator operating at `a predetermined frequency, a pair of cavities resonant at said frequency coupled to said generator, semiconductors mounted in each of said cavities, an optical opening in at least one of said cavities for permitting the radiation to be detected to impinge upon said semiconductor, waveguide means coupled to said cavities to receive microwave energy respectively therefrom in opposing phase relation, means yfor varying the relative amplitudes of the microwave energy thus transmitted to said waveguide means from said cavities to balance the same in the yabsence of said radiation, and detector means coupled to said waveguide means to detect the imbalance of the microwave energy received therein from said first and second waveguides.

9. A radiation detector comprising a microwave generator operating at a predetermined frequency, a pair of cavities resonant at said frequency coupled to said generator, semicondutcors mounted in each of said cavities for absorption of varying quantities of microwave energy dependent upon the number of electrons of said semiconductor in the conduction band, an optical opening in each of said cavities for permitting the radiation to be detected to impinge upon the semiconductors therein, a chopper in the path of said radiation to cause periodic interruption of the excitation of said semiconductors in a1- ternation at a predetermined modulation frequency, waveguide means coupled to said cavities to receive said microwave energy therefrom in opposing phase relation, and detector means coupled to said waveguide means to detect the imbalance of the microwave energy received therein from said first and second waveguides.

10. A radiation detector comprising a microwave generator operating at a predetermined frequency, a pair of cavities resonant at said frequency coupled to said generator, semiconductors mounted in each of said cavitiesv for absorption of varying quantities of microwave energy dependent upon the number of electrons of said semiconductor in the conduction band, optical openings in each of said cavities for permitting the radiation to be detected to impinge upon said semiconductors, chopper means in the path of said radiation to interrupt the eX- citation of said semiconductors in alternation at a predetermined modulatingfrequency, waveguide means coupled to said cavities `to receive said microwave energy therefrom in opposing phase relation, means for varying the amplitude of microwave energy transmitted to said waveguide means from at least one of said cavities to balance the energy received by =said waveguide means in the absence of radiation, and detector means coupled to said waveguide means to detect the imbalance of the microwave energy received therein from said first and second waveguides.

References Cited in the le of this patent UNITED STATES PATENTS 2,562,281v Mumford July 31, 1951 2,605,323 Samuel July 29, 1952 2,819,453 Cohn Ian. 7, 1958 2,832,045 Sharpless Apr. 22, 1958 2,844,737 Hahn et al July 22, 1958 2,844,789 Allen July 22, 1958 

